Sciencemadness Discussion Board

silica in organic solvents

Bender84 - 22-5-2020 at 07:21

Hello,

I'm trying to estimate silica content (as silicic acid) in organic solvent, which is 2-ethylhexanol (2-EH). The test I'm using is the photometric Merck test for determination of silica as silicic acid (in sulfuric solution silicate ions react with molybdate ions to form a yellow heteropoly acid. This is reduced to silicomolybdenum blue that is determined photometrically).

The problem is that this test is dedicated to water solutions only. I tried to make a water extract from the aforementioned solvent but the results I got were unrepeatable and inconclusive. Also when I do a second extraction with water of the same sample, I got a bit different result, so it is not like the whole silica - if present - is extracted from the solvent at the first approach (the 2-EH samples with distiled water in 1:1 ratio were shaked for 20 min).

My question is: having the nature of silica in mind, is it posssible that it is present in 2-ethylhexanol? Also can any of You recommend an approach to tackle this problem, i.e. how to estimate the silica content in 2-EH (if it makes sense to do so)?

Regards.

Sulaiman - 22-5-2020 at 13:46

could you evaporate (or distill off) the solvent to dryness then check for residue by weight ?

UC235 - 22-5-2020 at 16:15

Why do you expect silica in 2-ethylhexanol? Solubility should be roughly zero. Phosophomolybdic acid also readily forms from trace phosphate and is reduced to a bright blue compound. Are you sure you're not detecting this as contamination?

[Edited on 23-5-2020 by UC235]

Bender84 - 22-5-2020 at 22:57

Thanks for the answer!

The idea of 2-EH contaminated with silica came from a nitration tests where at the end of the reaction, apart from the reaction mixture which is 2-EHN and spent acids (sulphuric acid, nitric acid and water), I got a white, gelitnous residue. I centrifuged the residue, washed it with water, dried in oven and analyzed using FTIR. The spectrum was identified as SiO2. I am sceptical about this, but my supervisors are quite fixed on the idea that it is indeed silica or some sort of silicate present in the solvent.The problem occured when I tried to nitrate 2-EH from different manufacturer, so the only thing that has changed in the whole procedure is the alcohol. Still, I don't get repetable results when I am using the Merck test and the whole concept seems a bit far-fetched.

I tried to evaporate the solvent but didn't find any silica. Also I could not analyze the residue by disolving it in water due the fact that the Merck test should be done using plastic utensils and I had to evaporate the solvent on a watch glass. I will check the phosphates, though. Thank you!

Regards.

Bedlasky - 23-5-2020 at 06:41

Quote: Originally posted by UC235  
Why do you expect silica in 2-ethylhexanol? Solubility should be roughly zero. Phosophomolybdic acid also readily forms from trace phosphate and is reduced to a bright blue compound. Are you sure you're not detecting this as contamination?

[Edited on 23-5-2020 by UC235]


I use these Merck tests in job for detecting silicates in waters and these waters also contains phosphates. There isn't any phosphate interference. These reductions of molybdates and heteromolybdates strongly depend on pH. Molybdates, phosphomolybdates, silicomolybdates - each of this anions need different pH for reduction.

Bender84: Try water solution with Merk reagents (without reducing agent) - it should extract silicates from alcohol if there are present. Silicomolybdates are very stable complexes. After extraction just add reducing agent.

[Edited on 23-5-2020 by Bedlasky]